(1) Field of the Invention
The present invention relates to a field-effect transistor having a heterojunction gate structure.
(2) Description of the Prior Art
The field-effect transistor having heterojunction gate structure has been studied with the aim of improving the operable high frequency limit of transistor devices. The prior art field-effect transistor of this type employs an intrinsic or a slightly P-type GaAs layer grown on a semi-insulator GaAs substrate. Source and a drain regions are formed at a surface portion of the GaAs layer. On the surface portion between the source and drain regions is formed a compound semiconductor layer having an energy band structure by which a two-dimensional electron layer is induced on the surface portion of the GaAs layer. A gate electrode is formed on the compound semiconductor layer. An input signal is applied to the gate electrode to modulate the two-dimensional electron layer.
The gate electrode of the prior art is made of a metal or a conductive compound semiconductor doped with donor or acceptor impurities. Such a gate structure has a fixed difference in work function between the GaAs layer and the gate electrode, resulting in lower controllability of the threshold voltage. This causes difficulties in a circuit design having small power consumption, in the formation of complementary type P- and N-channel transistors and in the formation of a depletion type field effect transistor.